The present invention relates to controlling fluid suspension systems, and more particularly to controlling fluid pressure within the fluid suspension system of a vehicle.
Fluid suspension systems are known for providing a softer, more comfortable ride for a vehicle. Other common applications for fluid suspension systems include: raising or lowering a vehicle; leveling a vehicle that is under a load; leveling recreational vehicles parked on inclined surfaces; and altering the performance characteristics of a vehicle. Conventional fluid suspension systems include one or more pneumatic devices, such as air springs, connected between the vehicle axles and the vehicle chassis. Pressurized air from an air supply can be forced into or exhausted from one or more of the air springs to provide the vehicle with desired suspension characteristics. Such a system may be installed on a vehicle by the original equipment manufacturer, or they may be purchased as aftermarket products that are substitutes or supplements for conventional steel spring suspensions.
As mentioned above, fluid suspension systems may be used to adjust the ride height of a vehicle. In order to achieve and maintain a suspension ride height, control systems have been employed that implement closed loop algorithms with feedback. Feedback such as height control is employed where the suspension deflection is measured and air is either added or vented to inflate or deflate the air springs to achieve and maintain suspension height. Height control varies from mechanical height control valves to electronic height sensor-based electronic control systems. These systems operate by monitoring actual suspension height; once the suspension is at its target height, the valves close to stop inflation or deflation. This is referred to as “closed loop control”: direct measurement of height is used to “know” when target heights are achieved, and valves are directly actuated to start or stop inflation or deflation based on the measured height.
While height control systems may be desired for their accurate closed-loop control of height, these systems may suffer from the added cost and complexity of height sensors that are exposed to the aggressive under-vehicle environment around the tire. Rocks, snow, mud and debris may disable or damage the height sensors. It is also time consuming to mount height sensors on vehicles that have not been designed for them—it is especially labor intensive to install aftermarket height sensors on passenger cars where packaging is tight. Custom brackets specific to each vehicle are often fabricated and welded or bolted in place, and then the height sensor wiring is routed to each sensor. The height sensor install typically takes sixteen hours, with the rest of the control system taking less time—four hours typically.
Other closed-loop systems have been employed using pressure-based feedback. However, these pressure-based systems are either not cost effective or in some circumstances unable to obtain accurate pressure readings during inflation and deflation of the air springs. For example, accurate real-time reading of air spring pressure may be obtained inside the air spring, but doing so is generally not cost effective. Pressure sensors placed remote from the control system and within the air spring may be more expensive to implement, more prone to failure and increase installation time. In-line air pressure, on the other hand, may be more cost effective and robust and easier to install (no wiring harnesses out to air spring mounted pressure sensors that are subject to the aggressive under-vehicle environment). But in-line pressure feedback systems encounter issues with dramatic air pressure changes during inflation and deflation, due in part to the high compressibility of air. That is, the pressure sensor reads “in-line” pressure, where the pressure reading can nearly equal tank pressure (approximately 150 psi) when inflating and atmospheric pressure when deflating. This effect may mask the actual pressure within the air spring, causing the feedback control system to be less reliable. The valves may be closed (no inflate or deflate) for a few seconds to allow pressure to equalize in the air spring and in-line before an accurate pressure reading can be taken at the manifold, but continually adjusting pressure and then pausing for an accurate measurement increases the loop time.
Further, because air spring pressure may not directly correlate to vehicle suspension height, conventional pressure-based systems do not always achieve target suspension height. Vehicles experience variations of loading (people, equipment, supplies, towed trailers, etc); to accommodate this load variation and maintain a consistent height, the air spring pressure may be changed. But without knowing a correlation between suspension height, air spring pressure and loading conditions, the conventional pressure-based closed-loop system may be unaware of whether and how much to increase or decrease the pressure. For example, conventional systems have been programmed for operating with a specific vehicle make and model (e.g., a Honda Civic), but when used in connection with other vehicles, such conventional systems may not achieve target suspension height due in part to differences in suspension characteristics between vehicle makes, and sometimes among vehicles of the same model.